Familial influences on tests of mental and motor performance at ages eight months and four and seven years were examined by comparing correlation coefficients among monozygotic twins, dizygotic twins, full siblings, half siblings, and cousins within race, sex, and social class groups. A paper has been published on familial resemblance in infant mental development. The correlations between scores of twin and sibling pairs on the Stanford-Binet (age 4) and Wechsler (age 7) intelligence tests suggested a greater genetic influence than was found for infant test scores. Correlation between scores of half-siblings, rarely reported, were consistent with predictions from a polygenic model of inheritance of intelligence. An investigation of relatives of children with IQs in the retarded range suggested genetic influences on the etiology of mild retardation among whites but not among blacks. This project is now terminated.